Material Sensing Using RAIN RFID Tags With Auto-Tuning Capabilities

In this paper, we demonstrate how the power-on-tag-forward (POTF) and reverse (POTR) resonance frequencies can be estimated simply by measuring changes in the autotune (AT) code of RAIN RFID chips capable of making capacitance adjustments for enhanced antenna impedance matching. We show how this approach allows us to reliably estimate these characteristic frequencies — and, by extension, the dielectric and magnetic properties — of objects using a simple reading of the AT state values in the chip memory. Therefore, we eliminate the need for full POTF and/or POTR curve measurement and the need for read distance estimation and environmental calibration. The proposed method shows repeatability using 6 diverse RAIN RFID tags with T-matched antenna designs and self-tuning ICs, deployed on 7 dielectrics and 1 magnetic material. Current limitations and future research directions are also discussed.